She is referring to the poisonous effects that toxins have on people. For example, pesticides get into drinking water and make people sick, even killing some. Pesticides and agricultural chemicals get into our food supply. We eat fruits and vegetables sprayed with DDT and it makes us sick. It causes cancer and all sorts of other horrible conditions. Carson was trying to get humans to see that we are an important part of the natural world and that we cannot keep poisoning the environment. Humans should be better stewards of the natural world, seeking out better and less harmful ways of controlling insects. When we use toxins, we are upsetting the balance of nature. She believes it is arrogant and dangerous for humans to think they should control nature instead of coexisting with nature.